bwebel

As I understand, the current 12 speed SRAM flattop chains need to be paired with particular chainrings. Does anyone know of any such rings for the new FSA carbon tandem cranks that would work? Or any double rings for the standard 130 BCD? The only ones I'm finding seem to be single speed rings and that won't quite give me the range we need on a tandem, even with 12 speed rear. thanks, Baird

Chilepines

I just setup a new tandem with FSA SL-K cranks, modified to 46/30 front, and 10-42 11- speed cassette. Much easier to stick with 11 speed IMO and you could squeak even more range out of this type of set up.
That does not directly answer your question - any 2x FSA modular chainrings should work with the cranks.

chojn1

@bewebel,
What is the advantage of the flat top chains?
I had bought a set for my bike because it was the only 12 speed chain my local bike shop had at the time. I had so much problems with their shifing that I switched back to the regular chains when one was finally available.
CJ

bwebel

I do not think there is any actual advantage to flattop chains. SRAM slightly changed their specs when they brought out 12 speed so they wouldn't be compatible with other manufacturers. Seems a rather crappy move for a company that probably wouldn't exist if they hadn't sued Shimano for antitrust violations a couple of decades ago. I like the gaps in the SRAM 10x33 cassette, but if I can't get chainrings to work, then I won't be buying it. I guess that's kind of karma for the incompatibility that SRAM introduced in the first place.

Chilepines

What size chainrings are you looking for?
The FSA tandem crank comes with a 110 BCD spider, not 130. That spider is asymmetric so I think only FSA specific rings will work. FSA makes a 46, 48, 50, 52 and 53 tooth modular ring which can then take several different inner rings - also specific to FSA as they have a 90 BCD.

bwebel

Going with a 10t cog, I'd probably put a 48 or 50t ring on, but I haven't found any of the FSA-specific rings that will work with the flattop chain, nor an FSA spider that would allow other chainrings to work. (I did find some 5 arm 110 BCD rings that would work with the flattop chains).
I asked about the 130 particularly because I have an older FSA tandem crankset that I could repurpose rather than buying new, if that could be made to work. I think my interim solution is going to be just use the 12 speed Shimano cassettes, whenever they are available, and hope the cog spacing is close enough to work with the SRAM shifters/derailleurs.

Chilepines

I found something that explains the flat top chain issue which makes sense. It's a compatibility issue with the RD not the chainrings.
"The Flattop road chain does NOT work in combination with the Eagle AXS rear derailleur. You have to use an Eagle chain. The reason is that the Eagle derailleur has standard pulleys, but the Flattop can only wrap around a pulley with teeth in one direction (the flat size doesn’t have enough space for full teeth. SRAM road AXS rear derailleurs have a special upper pulley to deal with the Flattop chain, but Eagle rear derailleurs do not." You should also be able to a Shimano 12 speed chain on a SRAM road cassette and RD. So you could go with your SRAM 10-33 and just get a Shimano chain.
BTW the FSA SL-K tandem crank is quite nice in how modular it is - you just need a tool to remove the locking and can then easily change out spiders or rings.

chojn1

@bwebel,
So, why not skip the flatop chains if there is no advantage. Sram regular non-flattop 12 speed chains will work fine with your 10-33 cassette and what ever chain rings you have. As Chilepines stated, Shimano 12 speed chains would also work.

bwebel

I do not believe Shimano 12 speed chains will work with flattop cassettes. It seems to be more than just incompatibility with the rear derailleur pulleys. From https://www.velonews.com/gear/techni...compatibility/
"the tooth profiles on the SRAM 12-speed 10-33 (or 10-26 or 10-28) road cassette is cut to mate with the larger rollers and narrower inner width of the 12-speed Flattop road chain; an Eagle or aftermarket 12-speed chain would skip on that cassette."

I think the roller size difference is close enough, and chainrings soft enough, that the flattop chain may work and slowly wear down the chainring to fit, but I'd rather have something designed to work out of the box. But the fact that SRAM designed this incompatibility for something as basic as a chain is annoying me enough that I may just go to another manufacturer.

wtandem

Carbon-Ti makes 110 BCD chain rings compatible with SRAM flattop, but there are at least two 110 mm BCD 4 bolt standards, and the rings are not compatible with the FSA tandem crankset. Fortunately I was able to return the rings and the FSA crankset. The only compatible crankset I found was Lightning, which makes a 107 mm BCD bolt circle spider compatible with SRAM. I used Force rings ( Red only comes one piece with the crankset ). Certainly, would have been nicer if there was more choice for tandems. I do really like the stock 46x33 with 10x36 shifting and spacing. I did not realize the FSA Tandem crankset has replaceable spiders. Maybe you can find a machinist to make the right spider.

bwebel

The Lightning are nice cranks, but not sure I can justify the $$$. Only other quasi solution I found was regular 5 bolt 110 rings from Praxis, which are flattop compatible, but are technically out of spec as they all are 16 tooth gaps. And given SRAM's challenges on front shifting, I don't know how well the AXS front derailleur would handle it. DaVinci does a 5 bolt 110 spider with their cranks, so that could work. Praxis also does a 53x39 130 BCD pair of rings, but a 53x10 combo would be really bigger than I could ever see needing. Might as well stay with 11 speed and and 11t smallest cog.

chojn1

I am currently running SRAM Rival 12 speed 10-36 road cassette, Rival AXS rear derailer, and eagle 12 speed chain - shifts perfectly.

wtandem

What tandem crankset, chainrings, and front derailleur are you using?

chojn1

@wtandem,

I am using the Lightning cranksets. I have both the 110 and 130mm spiders which allow for pretty much any chain ring I want to use. At home in the flatland of Texas, I use the 130mm spider with a 48t narrow wide chain ring and no front derailleur. With a 10-36t 12 speed cassette, we can pedal up to 40mph. My wife don't really want to pedal any faster than that. Omitting the front derailleur and switching to the SRAM wireless etap significantly simplified my drivetrain. The shifting has been flawless.

When touring a hilly area, I'll put on the 110 spider and 36t chain ring. Top pedaling speed is reduced to 30mph, but the low gears allow us to comfortably pedal up most any hills fully loaded. We are usually not in a hurry on tours anyway. This set up also makes unpacking and assembling the bike a little more simpler.